Read an arbitrary memory location?

This is a discussion on Read an arbitrary memory location? within the Linux Programming forums, part of the Platform Specific Boards category; The problem: I am trying to create a small memory dumper. Sample code: Code: #include <stdio.h> #include <stdlib.h> #include <sys/mman.h> ...

  1. #1
    uint64_t...think positive xuftugulus's Avatar
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    Read an arbitrary memory location?

    The problem:
    I am trying to create a small memory dumper.

    Sample code:
    Code:
    #include <stdio.h>
    #include <stdlib.h>
    #include <sys/mman.h>
    
    int main()
    {
            int *x;
            int err;
    
            x = (int *)0xFF;
            err = mprotect(x, 256, PROT_READ);
            if(err)
            {
                    printf("mprotect failed. Memory can not be read.\n");
                    exit(1);
            }
            printf("%p[%x]\n", x, *x);
    }
    I though the call to mprotect should succeed when the program is run with super user permissions, but it doesn't.
    Any ideas?
    Code:
    ...
        goto johny_walker_red_label;
    johny_walker_blue_label: exit(-149$);
    johny_walker_red_label : exit( -22$);
    A typical example of ...cheap programming practices.

  2. #2
    Captain Crash brewbuck's Avatar
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    mprotect() can't be called on a region that was not obtained via mmap(). At least on Linux. Also, you passed a count of 256, which is not a multiple of the page size, so it could never possibly work. Also, 0xFF is not a multiple of 4096, so again, could never possibly work. Also, you know about virtual memory, right? 0xFF is not a physical address and won't be interpreted as one.

    Besides, 0xFF is in the null page, so you will never, ever be able to access or map that page.

    To look inside physical memory, just open /dev/mem and start looking.

  3. #3
    uint64_t...think positive xuftugulus's Avatar
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    That's what i was thinking atm. Just pondered if there was a way through system calls.
    Code:
    ...
        goto johny_walker_red_label;
    johny_walker_blue_label: exit(-149$);
    johny_walker_red_label : exit( -22$);
    A typical example of ...cheap programming practices.

  4. #4
    Captain Crash brewbuck's Avatar
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    Quote Originally Posted by xuftugulus View Post
    That's what i was thinking atm. Just pondered if there was a way through system calls.
    No, /dev/mem is the only way to look at physical memory.

  5. #5
    uint64_t...think positive xuftugulus's Avatar
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    Code:
    #include <stdio.h>
    
    int main()
    {
            int x;
            FILE *m;
    
            m = fopen("/dev/mem", "r");
            if(m == NULL)
            {
                    printf("Could not open memory!\n");
                    return 1;
            }
            fseek(m, SEEK_SET, 0xFF);
            fread(&x, 1, sizeof(x), m);
            printf("&#37;x: %x\n", 0xFF, x);
            fclose(m);
    }
    This much simpler version works. Thanks!
    Code:
    ...
        goto johny_walker_red_label;
    johny_walker_blue_label: exit(-149$);
    johny_walker_red_label : exit( -22$);
    A typical example of ...cheap programming practices.

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